1. Field of the Invention
The present invention relates to high frequency integrated circuit chip assemblies and more particularly concerns an improved method of mounting an integrated circuit chip to a main high frequency assembly.
2. Description of Related Art
High frequency circuitry that has been developed and used for many purposes, such as amplifiers of high frequency radar and communication systems, for example, often employs a substrate carrying high frequency microwave circuitry. The main high frequency assembly may include among its components a monolithic microwave integrated circuit chip (MMIC) capable of handling signals of very high frequencies, such as, for example, in the order of 18 to 20 gigahertz, or higher. The high frequency main assembly incorporates many circuit components which are individually fabricated, assembled and interconnected on the main assembly substrate. Components of the main assembly substrate are secured by soft or relatively low temperature soldering, whereas a MMIC chip requires hard solder or high temperature soldering. Required use of hard solder causes the MMIC chip to be difficult to attach and/or re-work in the main assembly, because use of the high temperature solders may degrade or even melt the softer soldering of the other components. Moreover, use of the soft solder requires flux, which is detrimental to a nearby MMIC chip already installed. Further, MMIC chips are very fragile devices, having a dimension of between 2.times.2 millimeters, or smaller, up to about 4.times.4 millimeters, or larger, and a total thickness in many instances no more than 0.004 inches. Thus, in the past, mounting of the MMIC chip on the main assembly has required the fragile chip to be exposed to work done on the other components, creating high potential for scrapping of the entire assembly. The potential dangers to the MMIC chip not only place this chip at risk, but also may result in damage to other electrical devices within the assembly and possibly to the carrier or header of the device.
In addition, the MMIC chip, as other chips, is required to be tested and to have certain of its circuit functions burned in. In the past this has generally been done after assembly of the very fragile MMIC chip on the main assembly. Once a MMIC device is successfully installed into a main assembly, the entire assembly is required to go through testing, screening, and burn-in procedures which apply only to the MMIC device. This creates a potential for damage to portions of the assembly other than the MMIC device. Testing failures discovered after assembly of the main system can be expensive and time consuming to repair, often requiring removal of different ones of the components, and the very thin and fragile MMIC chip.
Further, it has been discovered that hydrogen in sealed microelectronic packages has a detrimental effect on the performance of gallium arsenide MMIC devices, thereby requiring alternate methods of packaging. One such method that has been adopted is to vent the chip package so as to allow emerging hydrogen to escape before reaching detrimental levels. This approach, however, precludes the use of silver loaded epoxies for attachment of the MMIC chip because of silver migration caused by moisture levels in the air.
Assembly of a microwave or micro-strip system or circuit which contains a MMIC chip has been accomplished, in the past, in (chronological) series with the delivery of a working MMIC chip because of the fact that the MMIC chip must be installed before screening and lot qualification tests can be performed.
Accordingly, it is an object of the present invention to provide for installation of a fragile chip by arrangements and methods that avoid or minimize above-mentioned problems.